1. Technical Field
The present invention relates to a demodulation method and a demodulation device for demodulating data of a navigation message modulated by the BPSK (Binary Phase Shift Keying) modulation system.
2. Related Art
As a positioning system utilizing a position signal, the GPS (Global Positioning System) is widely known and used for a receiving device installed inside a mobile phone, car navigation system and the like. In the GPS, the positions of plural GPS satellites, the pseudo-distance from each GPS satellite to the receiving device and the like are found, using clocked time of the GPS receiver, and ultimately the position is computed.
A GPS satellite includes information about satellite orbit, time and the like into a navigation message and transmits the information at a bit rate of 50 bps (bits per second). A navigation message includes a master frame. A master frame includes 25 1500-bit frames.
The receiving environment of GPS satellite signals varies depending on the position of the receiving device and the like. In a weak electric field environment, since the signal intensity of a received signal is very weak, there is a possibility that a change in the bit value (bit transition) of a navigation message cannot be detected accurately, causing an error in demodulation of the received signal. Therefore, as a technique for determining whether a received signal is correctly modulated or not, error detection is known in which whether each word included in each sub-frame of a navigation message is correctly demodulated or not is examined, using an error detection bit included in the navigation message. However, in a weak electric field environment, as described above, since the signal intensity of the received signal is very weak, a change in the bit value (bit transition) of the navigation message cannot be detected accurately. Therefore, in some cases, whether a bit determined as having an error really has an error, and whether the other bits really have no errors cannot be determined accurately. Thus, a technique in which the bit value of each bit is decided by adding a bit transition if there is a bit determined as having an error is known (see, for example, U.S. Pat. No. 5,768,319).
However, U.S. Pat. No. 5,768,319 has a problem that a bit transition is added to all the 1500 bits, that is, even to the bit of a word that is not used for positioning, thus leasing to an increase in memory volume in demodulation processing.